A combo is a deck that is not big money, engine, rush or slog.
You are starting from a point so wrong that it's difficult to describe why this is wrong. But I will try anyway. In bold below I point out the distinguishing characteristics of a Combo strategy, an important thing to note about deck archetypes and how to think about them, and why it's a bad idea to use definitions that no one else agrees with.
Deck archetypes are a tool used by players to classify and understand the different types of decks we can build in Dominion. Each archetype has certain characteristics that make it unique. For example, Big Money involves buying mostly money and only a few actions. Engine involves getting a lot of actions and playing most or all of your deck every turn. Rush involves quickly getting lots of a specific card (IGG) or class of cards (like Actions for Vineyards), getting a point margin over your opponent, and quickly ending the game. Combo involves obtaining or using
two or more cards or events whose synergy is both unique (or nearly so) and so strong that it enables you to establish a point margin over your opponent and win the game, either by giving huge buying/gaining power or by generating massive amounts of VP. For example, Hermit/Market Square, Counting House/Travelling Fair, Scavenger/Stash, Bishop/Fortress. An important thing about combo decks is that at least one of the cards/events
cannot be switched out with any other card/event and still be effective.
So, isn't the standard Bishop Golden deck a combo? It's certainly not an Engine, since you aren't doing lots of different things on your turn. It does actually have some characteristics of Big Money (buy only one or few actions, otherwise money and VP), Rush (once you get the Golden Deck and a lead your opponent is on a timer to get the lead back), Slog (if you trash Golds instead of Provinces), and Combo (no other card can fill the Bishop slot). But
deck archetypes are not rigid categories into one of which every single possible Dominion strategy fits. A strategy does not have to fit exactly one of the archetypes. It can be a mix of several, or something that comes up rarely enough that people haven't bothered to create a new archetype describing it.
Under your definition a "Good Stuff" deck would be a combo. A newbie deck full of random Action cards with no synergy would be a combo. And as you said above, you consider keeping your starting deck and not buying anything to be a combo strategy. This is completely at odds with the general English meaning of the word "combo".
The main reason why it is bad to use your definition is that no one else uses it, and so by insisting on using it, without proactive explanation, you create an impediment to greater understanding.